The invention relates generally to chemical mechanical polishing (CMP) systems, and more particularly to a system and method for polishing semiconductor wafers using polishing pads having diameters that are smaller than the diameter of the wafers being polished.
During a fabrication process of a high density multi-layered semiconductor device, one of the most important processing steps is planarizing a layer of a semiconductor wafer by removing uneven topographic features of the wafer. The layer planarization allows patterns that are subsequently formed above that layer to be more uniform. In the case of conductive patterns, the planarization of the underlying layer reduces the probability of electrical shorts between the conductive patterns, which is a growing concern as the density of microelectronic circuitry included in a semiconductor device is progressively increased.
Chemical mechanical polishing (CMP) is a well-accepted technique to planarize a layer of a semiconductor wafer during the fabrication process by chemically and mechanically removing uneven topographic features of the wafer. A conventional CMP technique involves polishing the surface of a wafer with a rotating polishing pad using a slurry of colloidal particles in an aqueous solution. The slurry promotes planarization of the wafer surface by producing a chemical reaction with the wafer surface and by providing abrasives to xe2x80x9cgrindxe2x80x9d the wafer surface with the polishing pad. Typically, the polishing pad used for CMP is larger than the wafer being polished. That is, the diameter of the polishing pad is greater than the diameter of the wafer. Thus, the entire surface of a wafer is usually polished by a single polishing pad. For many CMP techniques, the polishing of the wafer surface is followed by a buffing step, during which the wafer surface may be further polished using a slurry containing finer abrasive particles. After the buffing step, the wafer is then cleaned and dried, which completes the CMP process. Thus, a typical CMP system includes a polishing unit, a buffing unit, and a cleaning unit.
In order to increase the throughput of planarized wafers, CMP systems have been developed that can simultaneously polish and/or buff multiple semiconductor wafers. The polishing of multiple wafers is accomplished by using a single large polishing pad to collectively polish the multiple wafers, or a number of different polishing pads to individually polish the wafers. Similarly, the buffing of multiple wafers is accomplished by using a single large buffing pad, or a number of different buffing pads.
A concern with these conventional CMP systems is that the amount of polishing at different regions of a wafer surface by a large polishing pad cannot be controlled with any significant degree of precision, which may result in a non-uniform wafer surface.
Another concern with the conventional CMP systems is that the footprint of the systems tends to be large, partly due to the large polishing pads. In addition, the increase in throughput is not as significant for planarization of semiconductor wafers that require short polishing periods.
In view of the above concerns, there is a need for a system and method for chemically and mechanically polishing semiconductor wafers that provides more precise control of the amount of polishing at different regions of a wafer surface, increased throughput for short period planarization, and reduced footprint for the system.
A system and method for chemically and mechanically polishing surfaces of semiconductor wafers utilizes multiple polishing pads having diameters that are smaller than the diameter of the wafers to simultaneously polish a given semiconductor wafer. The use of these smaller-sized polishing pads can significantly reduce the footprint of the system. Furthermore, the simultaneous polishing of the wafers by the multiple smaller-sized polishing pads can significantly increase the throughput for short period planarization. In addition, by independently controlling the lateral movement, the vertical movement and the rotational speed of each of the polishing pads during polishing, the system and method can more precisely control the amount of polishing at different regions of a wafer surface.
In one embodiment, a system in accordance with the present invention includes a rotatable platform that provides support for an object to be polished, a number of rotatable polishing pads, and a movement mechanism for independently moving each of the rotatable polishing pads laterally across a surface of the object. The rotatable polishing pads includes at least one pad having a surface area smaller than the surface area of the object to be polished. The movement mechanism is configured such that at least two rotatable polishing pads can be positioned over the surface of the object to simultaneously polish the object.
In another embodiment, the system includes a scanning mechanism for scanning each of the rotatable polishing pads laterally across the surface of the object about a fixed axis, instead of the movement mechanism. The scanning mechanism is also configured such that at least two rotatable polishing pads can be positioned over the surface of the object to simultaneously polish the object. In this embodiment, the scanning mechanism may include a mechanical arm having the rotatable polishing pads. The mechanical arm is configured to pivot about the fixed axis such that the rotatable polishing pads can be positioned over the surface of the object to simultaneously polish the object. The mechanical arm may include a first section and a second section. Each of the mechanical arm sections has one of the rotatable polishing pads. The second section of the mechanical arm is configured to pivot about the end of the first second of the mechanical arm.
In either embodiment, the movement mechanism or the scanning mechanism may include a first mechanical arm having a first rotatable polishing pad and a second mechanical arm having a second rotatable polishing pad. Each of the mechanical arms is configured to pivot about an arm axis such that the object is scanned by the first and second rotatable polishing pads when the first and second mechanical arms are pivoted. In addition, the movement or scanning mechanism may further include a controller that controls pivoting movements of the first and second mechanical arms. The controller is configured to control the first and second mechanical arms to position both the first and second rotatable polishing pads over the surface of the object so that the first and second rotatable polishing pads can simultaneously polish the object.
In either embodiment, the system may also include a secondary rotatable platform that provides support for a second object to be polished. In addition, the movement or scanning mechanism may include a multi-object mechanical arm having a multi-object rotatable polishing pad, which has a surface area smaller than the surface areas of the object and the second object. The multi-object mechanical arm is configured to pivot about an arm axis such that the multi-object rotatable polishing pad can scan the object and the second object to polish both the object and the second object when the multi-object mechanical arm is pivoted. Furthermore, the movement or scanning mechanism may also include a first primary mechanical arm having a first rotatable polishing pad and a second primary mechanical arm having a second rotatable polishing pad. The first primary mechanical arm is configured to pivot such that the object is scanned by the first rotatable polishing pad when the first primary mechanical arm is pivoted. The second primary mechanical arm is configured to pivot such that the object is scanned by the second rotatable polishing pad when the second primary mechanical arm is pivoted.
In either embodiment, the system may also include a rotational drive mechanism that is operatively coupled to the rotatable polishing pads to individually rotate each of the rotatable polishing pads. The rotational drive mechanism is configured to independently control the rotational speed of each of the rotatable polishing pads. The system may also include a vertical drive mechanism operatively coupled to the rotatable polishing pads to individually move each of the rotatable polishing pads along a vertical direction. The vertical drive mechanism is configured to independently control the pressure being applied to the object by each of the rotatable polishing pads.
In one embodiment, a method in accordance with the present invention includes the steps of providing an object to be polished, positioning a number of rotatable polishing pads over a surface of the object, and simultaneously polishing the object with the rotatable polishing pads. The step of positioning a number of rotatable polishing pads includes independently moving each of the rotatable polishing pads across the surface of the object. The rotatable polishing pads include at least one pad that has a surface area smaller than the surface are of the object.
In another embodiment, the step of positioning a number of rotatable polishing pads includes moving the rotatable polishing pads by pivoting at least one mechanical arm having at least one of the rotatable polishing pads, instead of including independently moving each of the rotatable polishing pads across the surface of the object.
In either embodiment, the step of simultaneously polishing the object includes individually controlling the lateral speed of each of the rotatable polishing pads across the surface of the object to control the amount of polishing by each of the rotatable polishing pads at different regions of the surface of the object. The step of simultaneously polishing the object may also include individually controlling the rotational speed of each of the rotatable polishing pads. The step of simultaneously polishing the object may also include individually controlling the downward pressure of each of the rotatable polishing pads on the object.
In either embodiment, the method may further include the step of conditioning a particular rotatable polishing pad on a pad conditioner, which is situated adjacent to the object such that the particular rotatable polishing pad can contact both the object and the pad conditioner. The method may also include the step of supporting a particular rotatable polishing pad on a supporting structure, which is situated adjacent to the object such that the particular rotatable polishing pad can contact both the object and the supporting structure.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrated by way of example of the principles of the invention.